The present invention relates to a communication device which is provided with pre-distortion to compensate for distortion introduced by components in the device, and a method of such pre-distortion.
In particular, the present invention relates to the compensation of distortion by components in a transmitter.
Components in the transmitter cause an information signal to be distorted prior to transmission to differing degrees. Two major factors affect this distortion, namely ramp up and ramp down, and component tolerances. Whilst individually such distortions may not affect the signal to be transmitted significantly, there is a possibility that the superposition of such signals is unacceptable as it is outside the limits defined by the appropriate telecommunication standard. For example, such distortion may cause a signal in the frequency domain to extend outside the bandwidth defined by a standard.
There are three ways in which compensation of such distortion may be provided. Firstly, during manufacture, each device can be measured, and the devices not meeting the necessary requirements can have the offending components altered. As will be appreciated, this increases manufacturing costs, and results in a reduction in yield over a given time. Secondly, active pre-distortion can be provided, for example on the basis of feedback from the output of a component to be compensated for. However, this increases the complexity of the transmitter and the power consumption. Thirdly, a filter may sometimes be used to compensate for distortion by certain components. However, this is not possible for cases where the distortion varies over a frequency range such as in non-linear devices (for example non-linear power amplifiers).
When designing telecommunication systems, there are a number of modulation schemes which may be selected from. Each one will have its own particular advantages and disadvantages. For example, GMSK (used in GSM) is spectrally inefficient but power efficient, whereas on the other hand QPSK (used in CDMA systems) is spectrally efficient but power inefficient. Distortion by components in the transmitter of a communication device can reduce the benefits of these schemes in practice. For example, GMSK was selected to be used in GSM on the basis that it provides a constant amplitude, and is thus power efficient. However, in practice, distortion of the signal by components in a GSM transmitter will cause variation in the amplitude envelope and thus a variation in power efficiency, so negating at least to some extent the advantages for which GMSK was chosen.